Talk Morgan Forums Morgan Sports Cars Morgan Three Wheeler & Super 3 Revs not going to idle

To avoid a permanent headache due to the M3W electrical problem I’m taking a few days off searching the cause.
But the problem continues to occupy me because a solution must be found.

As Richard pointed out in an earlier post on 8/11, you can see that YR and YB (the two wires in question to the wheel speed sensor) via the chassis/engine connector are routing via a joint bomb to many of the sensors on the engine.
The idling problem is solved when I disconnect these YR/YB wires at the speed sensor.

I have already swapped the engine sensors from the other car, but I cannot swap the plugs because they are on the loom.

If the problem needs to be sought in the engine loom, that needs to trigger the engine control light up every time there is high idling. Or is that wrong thinking?
Or is this only visible when using a computer with Pro Tunes software?

This could point to the problem being in the wiring from the speed sensor to the joint bomb. Perhaps the solution is to cut YR and YB and remake the wiring from the speed sensor plug to the bomb? RIchardV6 does this sound sensible?

Please note that my suggestion above Laurens was to interrupt only the yellow/white speed signal wire which normally runs from speed distribution bombtail to the chassis loom plug to benefit only the ECU. I imagine it could be easily separated out at this bombtail leaving the other wires to distribute the speed signal to speedo and TSM. I see no reason to suspect the yellow/red and yellow/black wires on loom which appear to be common reference lines, given you have reported the car runs fine with sensor plug removed, and their loom wiring still in place. Furthermore a short to ground on either would compromise the several associated sensors. In any case it would be difficult to achieve without cutting the wires!

Ref Chris and Bruce's suggestions, I suppose a short to ground on either of the reference wires on the sensor itself could compromise (although doubt engine would run at all) but I understand you have already tried a completely different speed sensor from your other car to no avail, with intermittent idle remaining.

It would only be a temporary fix but I would be interested to learn the outcome when you are able and willing to try this, and it would hopefully leave your car driveable.

Sincere apologies Chris, I failed to fully read Laurens last post and was fixating on what he had previously told us. So now yes your suggestion sounds sensible although Laurens confirms previously he had tried the working speed sensor from his other car with no change to idling fault, presumably by using its own plug into the chassis loom, which would surely rule out any fault on the original sensor

For clarity the image below shows the speed sensor connection top left and what I still presume is the raw speed signal distribution bombtail bottom right. This is unusual in the fact all four wires are differing colours.



Perhaps Laurens could re-confirm the engine idles correctly with the speed sensor disconnected. This would prove idling issues are not being caused by the YB or YR wires within the chassis loom as they route directly and exclusively to the engine loom connection.

Further investigation reveals that if the VSS is a hall effect device (very likely) then the three wires from it are power (YR), signal (YK) and signal ground (YB) and the VSS needs power to work. As the power and signal ground are common to several other sensors I think we can rule out any issues with the corresponding YR and YB wires from the sensor, connectors and looms. Clearly though Lauren's action of disconnecting the YB and YR wires at the sensor would disable it allowing the normal idle situation as he found, and given the current situation. This action would of course have killed the speed signal to the speedo and TSM as well, but just to be sure perhaps you can confirm that Lauren?

If above is correct then it brings us around in a rather large circle to my earlier untried suggestion of (temporarily) disconnecting just the speed signal to the ECU, YW wire out of bombtail leaving a working VSS to benefit speedo and TSM.

I have started a new search this week with again the strangest outcomes.
I was pleased that M3W Service France had an ECU and a TSM in stock, which they sent to me and allowed me to see what would happen when I swapped them.

I first did a test ride with the different ECU.
The changing of the ECU didn’t make any difference in behaviour regarding idling.
So now I’ve tried three different ECU, ’s there was not any difference.

I did a second test ride WITHOUT any TSM at all.
The car start and runs normally (except for idling). But no turn signals and no high beam light. The dipped beam was working. Instruments like Speedo are working normally.
Switching on hazard lights without keys runs the fuel pump, and you can start the car without keys.
High idling was still there, even without any TSM.

Changing to the different TSM gave strange results during my third test ride.
Depending on which kind of light you are turning on, it blows fuses from 7-9 or 10.
Intermittent high idling was still the case and happened as much as before with the “original” TSM.
As well as the starting of the fuel pump when you turn the hazard lights on.

This above behaviour was also the case when I used the TSM from my other car.

That means two “good” TSMs working normally in other cars give strange behaviour in this car.

As these other components don’t make it any better, I put back in the TSM and ECU, which belong to this car.

The “original” TSM let all things work normally. The lights on don’t blow any fuses, and all working normally.
Hazards light turning on let's run the fuel pump with all three TSMs

I disconnected one by one the instrument connectors C31 and C32.
Now the fuel pump didn’t start any more when turning on the hazard light.
The hazard light was blinking normally.
The lights were normal or not working, depending on which connector was disconnected.

I connected the “original” TSM and ECU again and went for a fourth test ride.
This showed that high idling was still intermittent, but the rest worked normally.

To ensure that disconnecting the VSS would solve the problems with the high idling, I went out for a last test ride.
Hurrah, hurrah, high idling is completely gone. Only no speed indication, and the turn signals don’t switch off automatically. The odometer also doesn’t work. But who cares? At least the engine now behave normally again.

As Richard suggested, the only next step will be cutting the yellow /white cable at the loom bomb.
But I wanted to keep it for the last as cutting is very easy, but repairing a cable in such an ugly place is a nightmare.

I've checked the voltage at the VSS connector near the wheel. The R/Y shows 4.99 volts, and the W/Y shows 4.88 volts.
This is the same as on my other car.

So much work and headaches are still there with all the above knowledge.

Well the good? news is you have a great degree of consistency with your findings Laurens. It would be normal for dip beam only with TSM disconnected as it energises the dip/main relay coil to give high beam when selected by stalk toggle. It appears your faulty car requires it's OE TSM. Certainly 2014 on cars had a different TSM, connector and loom, maybe there were other earlier undocumented variants of same?

Regarding removing VSS signal to ECU, I assumed the bomb tails were just twisted and or soldered wires with one of those screw on plastic cover/insulators, which would make disconnection easy. From your comments I guess not - maybe a large crimp?

''I've checked the voltage at the VSS connector near the wheel. The R/Y shows 4.99 volts, and the W/Y shows 4.88 volts.
This is the same as on my other car.' unquote.

The W/Y is probably the signal and will really need an oscilloscope to measure when the car is moving. There would be pulses so a meter should read 2.5V when moving. A duff TSM could pull that signal permanently high so you may see 4.88V when moving.

Proof is to disconnect the VSS signal going into the TSM, but leave it going to the ECU. Maybe you could jack the car up and run the rear wheel off the ground? I guess the VSS is on the gearbox output shaft somewhere.

Not sure where that info comes from Alister but the wire colour codes mentioned do not relate to those of the VSS (speed sensor) on the M3W


Note that Laurens has already proven the front wheel mounted VSS itself works ok with both the TSM (cancelling indicators below 10mph) and Speedo giving normal reading. Furthermore he has swapped out the VSS from his other car with no change. It's only when this same shared signal is connected to ECU (as normal) that his idle problems appear although TSM and Speedo still work correctly with speed signal.

@Laurens it may seem a desperate measure but I would be tempted to very thoroughly check the shared speed signal bombtail again. I know you said it looked ok in the crimp. I would particularly check the YW wire out of this tail as well for vibration induced partial fracture of copper strands within.

If nothing found then maybe try the suggested removal of this ECU speed feed YW wire. If driving tests reveal no negative issues with idle under a variety of driving conditions then it could be assumed it's not needed on your particular car.